The use of various organic acid metal salts as a reinforcing filler in golf balls is well known and has been described in numerous patents. Rubber compounds containing these salts exhibit high hardness and good rebound. One particularly useful salt is zinc dimethacrylate which is prepared from methacrylic acid. The salt has been conventionally added to the rubber during compounding.
Processes are also known for preparing the salt in situ during mixing with the rubber compound; however, there are many disadvantages. For instance, processability is not good because methacrylic acid is a liquid and it is corrosive, requiring the use of special equipment. Also, it is necessary to remove water, produced as a by-product; long mixing times are required; curing times are longer and, the properties of the compounds are variable because reaction control is difficult in situ.
With respect to the patent literature, U.S. Pat. No. 4,082,288, for instance, relates to the preparation of basic zinc methacrylate by milling under agitation methacrylic acid with a suspension of zinc oxide in a liquid medium such as water or a volatile organic liquid.
U.S. Pat. No. 4,100,182 relates to a method for preparing an adjuvant for an elastomeric composition which involves mixing methacrylic acid with zinc oxide in a liquid medium in the proportions required to form basic zinc methacrylate, removing the liquid medium and finely dividing the resulting reaction product. The reference teaches that the liquid medium may be water or a volatile organic liquid such as a hydrocarbon liquid or an alkanol.
U.S. Pat. No. 4,191,671 is directed toward abrasion resistant rubber compositions having improved flexing fatigue resistance which are prepared by heating and curing a mixture comprising a diene elastomer, an unpolymerizable carboxylic acid or metal salt thereof, an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, a divalent metal compound and an organic peroxide.
In U.S. Pat. No. 4,500,466, owned by the Assignee of record herein, a method is provided for the preparation of zinc methacrylate powder involving the reaction of zinc oxide and methacrylic acid in a liquid aliphatic hydrocarbon such as hexane.
U.S. Pat. No. 4,501,866 provides a continuous polymerization process for preparing high cis-1,4-polybutadiene including the steps of continuously feeding 1,3-butadiene in a hydrocarbon solvent to a reaction with a preformed .pi. allyl catalyst formed by mixing a carboxylated metal oxy borate compound with nickel or cobalt, an organoaluminum compound, an alcohol, a small amount of 1,3-butadiene and a hydrocarbon solvent and, a co-catalyst comprising a boron trifluoride complex with an alcohol.
U.S. Pat. No. 4,720,526 is directed toward a method of preparing cured rubber compositions having improved modulus properties. The method includes the steps of adding a metal methacrylate and a peroxide curing agent to a rubbery polymer and then curing the mixture.
Finally, Japanese Patent No. 63-223048 is directed toward a process which involves mixing rubber, a metal oxide, hydroxide or carbonate, acrylic acid and a solvent for the rubber. In this manner a rubber crosslinking agent, e.g., zinc diacrylate, is formed that is free from dust. Improved physical properties are imparted to the rubber products.
Despite the existence of metal salts of carboxylic acids and their widespread use in rubber polymer compounds for a variety of products including golf balls, the art has not provided a satisfactory method heretofore, for the uniform distribution of the metal salt throughout the polymer. Solid golf balls reinforced thereby have superior physical properties.